This invention relates to applying abrasion resistant metal oxide coatings to glass articles. More particularly, the invention relates to a method for preventing such coatings from being deposited on surface areas of the article where they are not wanted.
It is known that the abrasion resistance of glass articles, especially glass containers, can be significantly enhanced by forming a film on the glass surface of various pyrolysible metal compounds. Such coatings are commonly referred to in the industry as "oxide coatings", although they may not in fact be oxides and their exact chemistry is not understood. They are widely used in the commercial production of glass containers such as bottles, jars and the like.
It is well known that such a coating can be applied by contacting a glass container with vapor of various thermally decomposable or pyrolytic metal compounds while the glass article is at an elevated temperature that is sufficient to decompose the compound to form the oxide coating. This causes a film deposit to form on the glass surface which displays the enhanced scratch and abrasion resistance qualities. In one example of the usual practice, a bottle or jar at a temperature of about 700.degree. to 1300.degree. F. is exposed to and contacted by a vapor of a pyrolytic titanium compound such as tetraisopropyl titanate or titanium tetrachloride. At this temperature the vapor decomposes or pyrolyzes where it contacts the hot glass surface to form the film coating on the surface. The resulting film, which is very thin and almost invisible in ordinary light, is commonly referred to as a "TiO.sub.2 ", "oxide" or "titanate" film, although its exact composition is difficult to determine and is not known precisely. This coating enhances the resistance of the bottle to the abrasion which it inevitably experiences in manufacture, filling and use. Typical of the patent art disclosing the general method of forming abrasion resistant oxide coatings with various pyrolytic titanium or tin compounds are the U.S. patent to Deyrup, U.S. Pat. No. 2,831,780; Dubble et al., U.S. Pat. No. 3,438,803; and Scholes U.S. Pat. No. 3,420,693. Pyrolytic compounds of zirconium, and other metals are also well known for this purpose, and reference may be had to the art for detailed descriptions of such compounds and methods.
It is frequently desirable or necessary to coat the hot glass surface selectively, especially in connection with glass containers which are adapted to receive foodstuffs, e.g., baby food jars, beer or soft drink bottles, and the like, so as to prevent deposition of the film on the finish (or cap-engaging) area, as well as on the interior of the container. This is so because metal oxide coatings under certain conditions may have an adverse effect on some of the sealing characteristics of jars and bottles when metal caps are applied. In particular, metal oxide coatings may in some cases produce low seal security and high removal torque. These problems can be minimized by preventing the deposition of such coatings on the finish area of glass containers which are to receive such caps. It is also, of course, desirable to prevent deposition of the coating film on the interior of glass containers intended for foodstuffs, where the film might be in contact with foodstuffs therein. While deposition on the interior surface of narrow necked bottles such as beer bottles or soft drink bottles is not a severe practical problem, deposition on the inside of wide-mouth bottles such as baby food jars does present a potential problem without the method of this invention.
One method known to the prior art for controlling coverage of such metal oxide films on glass articles makes use of an air stream which is intended to sweep the treating compound vapor away from that area where the coating is not desired, e.g., the finish area and the inside surface area of a container, the objective being physically to remove the vapor before it contacts that area so that actual contact of the vapor with that area is prevented. This method may use dry or wet air. If dry air is used, the air simply mechanically sweeps the vapor away from the selected area prior to contact. Moisture or water vapor in the air stream tends to react with the treating compound, so as to deactivate it. Typical apparatus adapted to carry out that method is described in U.S. Pat. No. 3,690,289.
However, it is difficult to control the action of such a sweep-away air stream, because relatively high air velocities are required to insure that the thermally decomposable compound vapor will in fact be eliminated from that area where the coating is not desired, which makes it difficult to control the boundary of the selected area. Secondly, a relatively cool air stream is undesirable from the standpoint of avoiding a detrimental temperature differential between the different areas of the glass article, i.e., between the finish and body. Although hot air streams may be utilized, that adds to the expense of the air stream method. Third, it is difficult for a line operator to determine exactly where the air stream is impinging on the glass article, and the operator must more or less guess as to the proper positioning of the nozzle which directs the air stream against the glass article, for no visual indicia is available on line to indicate just where the air stream is impinging on that glass article and where the edge of the coated area is.
It has been the principal objective of this invention to provide a masking method for use where pyrolytic metal oxide films are being applied, which is easily controlled under production line circumstances, provides visual indicia to the operator for preventing the film from forming in those selected areas on the glass articles where it is not desired, and which provides a high order of masking for such areas.